Catalytic reforming is a well-established hydrocarbon conversion process employed in the petroleum refining industry for improving the octane quality of hydrocarbon feedstocks, the primary product of reforming being motor gasoline. The art of catalytic reforming is well known and does not require detailed description herein.
Briefly, in catalytic reforming, a feedstock is admixed with a recycle stream comprising hydrogen to form what is commonly referred to as a combined feed stream, and the combined feed stream is contacted with catalyst in a reaction zone. The usual feedstock for catalytic reforming is a petroleum fraction known as naphtha and having an initial boiling point of about 180.degree. F. (82.degree. C.) and an end boiling point of about 400.degree. F. (203.degree. C.). The catalytic reforming process is particularly applicable to the treatment of straight run naphthas comprised of relatively large concentrations of naphthenic and substantially straight chain paraffinic hydrocarbons, which are subject to aromatization through dehydrogenation and/or cyclization reactions.
Reforming may be defined as the total effect produced by dehydrogenation of cyclohexanes and dehydroisomerization of alkylcyclopentanes to yield aromatics, dehydrogenation of paraffins to yield olefins, dehydrocyclization of paraffins and olefins to yield aromatics, isomerization of n-paraffins, isomerization of alkylcycloparaffins to yield cyclohexanes, isomerization of substituted aromatics, and hydrocracking of paraffins. Further information on reforming processes may be found in, for example, U.S. Pat. No. 4,119,526 (Peters et al.); 4,409,095 (Peters); and 4,440,626 (Winter et al.).
A catalytic reforming reaction is normally effected in the presence of catalyst particles comprised of one or more Group VIII (IUPAC 8-10) noble metals (e.g., platinum, iridium, rhodium, palladium) and a halogen combined with a porous carrier, such as a refractory inorganic oxide.
In a common form, the reforming process will employ the catalyst particles in several reaction zones interconnected in a series flow arrangement. There may be any number of reaction zones, but usually the number of reaction zones is 3, 4 or 5. Because reforming reactions occur generally at an elevated temperature, and because reforming reactions generally are endothermic, each reaction zone usually has associated with it one or more heating zones, which heat the reactants to the desired reaction temperature and which supply the endothermic heat of reaction for the reaction zones. As a consequence of these considerations, the most common process flow through the train of heating and reaction zones in a 3-reactor catalytic reforming processes is as follows.
A naphtha-containing feedstock admixes with a hydrogen-containing recycle gas to form a combined feed stream, which passes through a combined feed heat exchanger. In the combined feed heat exchanger, the combined feed is heated by exchanging heat with the effluent of the third reactor. The heating of the combined feed stream that occurs in the combined feed heat exchanger is, however, insufficient to heat the combined feed stream to the desired inlet temperature of the first reactor. Consequently, after exiting the combined feed heat exchanger and prior to entering the first reactor, the combined feed stream requires additional heating. This additional heating occurs in a heater, which is commonly referred to as a charge heater, which heats the combined feed stream to the desired inlet temperature of the first reactor.
The combined feed stream then passes to and through the first reactor. Because of the endothermic reforming reactions that occur in the first reactor, the temperature of the effluent of the first reactor falls not only to less than the temperature of the combined feed to the first reactor, but also and more importantly, to less than the desired inlet temperature of the second reactor. Therefore, the effluent of the first reactor passes through another heater, which is commonly referred to as the first interheater and which heats the first reactor effluent to the desired inlet temperature of the second reactor.
On exiting the first interheater the first reactor effluent enters the second reactor. As in the first reactor, endothermic reactions cause another decline in temperature across the second reactor. Generally, however, the temperature decline across the second reactor is less than the temperature decline across the first reactor, because the reactions that occur in the second reactor are generally less endothermic than the reactions that occur in the first reactor. Despite the somewhat lower temperature decline across the second reactor, the effluent of the second reactor is nevertheless still at a temperature that is less than the desired inlet temperature of the third reactor. Consequently, the effluent of the second reactor passes through another heater, which is commonly referred to as the second interheater, and then passes to the third reactor.
In the third reactor, endothermic reactions cause yet another temperature decline, which is generally less than that across the second reactor, for the like reason that the temperature decline across the second reactor is generally less than that across the first reactor. The effluent of the third reactor passes to the previously mentioned combined feed exchanger, where the effluent of the third reactor is cooled by exchanging heat with the combined feed stream.
After a period of time in use, the catalyst becomes deactivated during the course of reforming reactions as a result of mechanisms such as the deposition of coke on the particles. Coke is comprised primarily of carbon, but is also comprised of a small quantity of hydrogen. Coke decreases the ability of catalyst to promote reforming reactions to the point that continued use of the catalyst is no longer practical or economical. At that point, the catalyst must be reconditioned, or regenerated, before it can be reused in a reforming process.
Numerous regeneration methods are in use commercially, and nearly all involve to some extent the combustion of coke from the surface of the catalyst. The particular method of regeneration that a specific reforming process employs depends on the design of the catalyst bed(s) in the reforming reactor(s). A commercial reforming reactor generally employs one of two different designs of catalyst beds: moving beds and fixed beds. In a moving bed, deactivated catalyst is withdrawn from the catalyst bed, while fresh or regenerated catalyst is added to the bed. Moving catalyst beds allow catalyst to be continuously moved from the reactor to an adjacent regeneration zone, regenerated, and moved back to the reactor. This is commonly referred to as continuous regeneration, although in practice it is often semicontinuous.
In contrast, fixed catalyst beds keep the catalyst stationary. When the catalyst in a fixed bed reactor becomes deactivated, the reactor is temporarily taken out of service while the catalyst is either regenerated in situ or else unloaded and replaced with regenerated or fresh catalyst. Two types of fixed bed regeneration methods are used commercially: cyclic regeneration and semi-regeneration. In the cyclic regeneration method, at least one or at most not all of the reactors are taken out of service at any one time and the reforming process continues in operation with the remaining reactors. After the deactivated catalyst is regenerated, the reactor is placed back in service, which in turn allows another reactor to be taken out of service for regeneration. In semi-regenerative reforming, the reforming process is temporarily stopped and all of the reactors are taken out of service simultaneously for regeneration. After the catalyst has been regenerated, all the reactors are placed back in service and the reforming process is resumed.
Commercial reforming process units that use the flow schemes and regeneration methods just described require a large investment of capital, and one of the major capital costs is that associated with the charge heater. As previously mentioned, the charge heater heats the combined feed stream to the desired inlet temperature of the first reactor. As also mentioned, the charge heater is typically needed because the heat that is transferred by the combined feed exchanger from the third reactor effluent to the combined feed is not sufficient to heat the combined feed stream to the desired inlet temperature of the first reactor. But, because of the capital cost of the charge heater, reforming processes are sought that eliminate the need for a charge heater.
It is known that a reforming process unit that uses the semi-regeneration method can operate without a charge heater. In one such unit, the combined feed heat exchanger comprises a plate type heat exchanger which heats the combined feed stream, and the heated combined feed stream then passes directly to a first reactor which contains a fixed bed of platinum-rhenium catalyst. The first reactor effluent then passes to a train of pairs of heaters and reactors that contain fixed beds of platinum-rhenium catalyst; that is to a first heater, a second reactor, a second heater, a third reactor, a third heater, and a fourth reactor. The fourth reactor effluent transfers heat to the combined feed stream via the plate type heat exchanger, and then passes to a product recovery section. The inlet temperature of the first reactor ranges typically from about 784 to about 849.degree. F. (418 to 454.degree. C.). The inlet temperatures of the second, third, and fourth reactors typically are within about 5.degree. F. (3.degree. C.) of each other, and are typically from about 81 to about 102.degree. F. (45 to 57.degree. C.) hotter than the inlet temperature of the first reactor, and usually vary over the range of from about 878 to about 939.degree. F. (470 to 504.degree. C.). The molar ratio of hydrogen per hydrocarbon feedstock is typically between 4 and 6.
It is also known that a reforming unit that uses the continuous regeneration method can operate with different feed inlet temperatures for each of the reactors. Typically, such a unit has a train of three, four or five pairs of heaters and reactors that contain moving beds of catalyst, but many of the various possible combinations of different inlet temperatures, which together form what is usually called the temperature profile of the unit, are perhaps best illustrated with a three-reactor unit. If the inlet temperatures of all three reactors are the same, then the temperature profile is commonly called flat, which is the profile that is most frequently employed in reforming units using continuous regeneration. If the inlet temperature of the first reactor is less than the inlet temperature of the second reactor, which is in turn less than the inlet temperature of the third reactor, then the profile of the reactor inlet temperatures is usually said to be ascending. If the first inlet temperature is more than the second inlet temperature, which is more than the third inlet temperature, then the profile is normally called descending. If the second inlet temperature is more than both the first and third inlet temperatures, then the profile is often said to resemble a hill. If the second inlet temperature is less than both the first and third inlet temperatures, then the profile is frequently said to look like a valley. Thus, in the ascending and hill profiles, the inlet temperature of the second reactor is greater than the inlet temperature of the third reactor.
The most common reason for operating with a non-flat (i.e., skewed) reactor inlet temperature profile is to allocate the required heat duty among the heaters in the heater-reactor train. Ideally, all of the heaters are individually delivering heat at approximately the same percentage of their individual design duties. When each heater is operating at the same percentage of its design duty as any other heater in the train is operating as a percentage of that other heater's design duty, then the heater duties are said to be "balanced." Of course, a heater should not, as a general rule, be operated in excess of its design duty, that is the percentage should generally be less than or equal to 100%. A flat profile could result in imbalance of the operating duties of the heaters in the train, if some of the operating variables such as feedstock quality or throughput differ significantly from their design values, or if flow maldistribution or mechanical problems causes the performance of a reactor to fall significantly below its expected performance.
An illustration of attempting to balance heater duties in a commercial continuous reforming process by skewing reactor inlet temperatures is described in the article by Richard Lee, et al. entitled "Reforming Processes, Maximizing Profitability," which begins at page 151 in Volume 47 of the Encyclopedia of Chemical Processing and Design, edited by John J. McKetta and published by Marcel Dekker, Inc., New York in 1994. In the example in the article by Lee et al., a valley-shaped profile of reactor inlet temperatures is recommended, where the inlet temperatures of parallel reactors 1 and 2 are the same and greater than the inlet temperature of reactor 3, which is less than the inlet temperature of reactor 4. Reactor 4's inlet temperature may be the same as or less than that of reactors 1 and 2. The largest difference between the reactor 1, 2, and 4 inlet temperatures and the reactor 3 inlet temperature is 26.degree. F. (14.degree. C.). The Lee et al. article also teaches that the magnitude of the differences between the gasoline range product (C.sub.5 + yield) when running an equal (that is, flat) reactor inlet temperature profile versus a staggered (that is, skewed) reactor inlet temperature profile is expected to be no more than 0.5% of feed.
Operating a reforming process without heating between the combined feed heat exchanger and the first or lead reactor has certain benefits which, however, are incapable of being realized in a reforming process unit that uses the semi-regeneration method. Therefore, reforming processes are sought that are capable of achieving more fully all the possible benefits and advantages of operating without a charge heater.